In wired communications, the conditions of the transmission medium do not generally change. However, in wireless communication, the conditions of the communication medium, i.e. the communication channel, may vary significantly over time as well as space.
Link adaptation allows transmission parameters used for communication to be adapted to the channel conditions of the link in order to optimally exploit the potential of the communication channel, usually to provide high data transfer rates along with low bit error rates.
For example, in a wireless communication system, such as a cellular or equivalent system, employing link adaptation for the downlink, the modulation and coding scheme and/or other signal and protocol parameters used by the access point for communication with the mobile terminal can be varied and is typically selected in dependence on the downlink channel quality. The channel quality is thus measured at the mobile terminal and a corresponding link mode is typically signaled in a link mode field to the access point, which adapts the modulation and coding scheme according to the signaled link mode. This means that the modulation and coding scheme can be optimized to the channel conditions, leading to a considerable improvement of the downlink channel throughput.
However, there exist many wireless communication standards, such as the IEEE 802.11 group of standards, which do not have any explicit signaling mechanism for link adaptation.
IEEE 802.11, for example, denotes a set of Wireless LAN standards developed by a working group of the IEEE LAN/MAN Standards Committee (IEEE 802). The IEEE 802.11 WLAN standard enables communication between stations in Infrastructure and Infrastructure-less systems. The former is denoted Basic Service Set (BSS), whereas the latter is called Independent BSS (IBSS) or ad hoc network mode in the IEEE 802.11 jargon. The 802.11 family currently includes six over-the-air modulation techniques that are all based on the same basic Medium Access protocol. The original standard, sometimes referred to as 802.11 legacy, defines Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) as the basic media access method. This has the advantage of improving the reliability of data transmissions under diverse and adverse channel conditions.
A general weakness of the original specification was that it offered so many choices that interoperability became a major challenge.
In 1999 the IEEE 802.11 standard was extended with a new physical layer based on Orthogonal Frequency Division Multiplexing (OFDM) and called IEEE 802.11a, enabling up to 8 different rates to be used. The maximum raw data rate is 54 Mbit/s, and the data rate can be reduced to 48, 36, 24, 18, 12, 9 and then 6 Mbit/s if required. IEEE 802.11a operates in the 5 GHz band, and uses 52 OFDM sub-carriers, 48 for data and 4 for pilots. Another standard called IEEE 802.11b for the 2.4 GHz band was also standardized enabling up to 4 different rates to be selected. In enabling the rate extensions, it was however left entirely up to the implementers to design an algorithm selecting the most suitable and appropriate rate at every instance.
More importantly, no mechanism for exchanging link adaptation messages between pairs of communicating stations was defined. Consequently, a station may merely perform link adaptation based on success or failure of earlier sent messages. This is an inefficient, slow and an unreliable method for link adaptation.
There is thus a general need to provide efficient support for link adaptation in wireless networks, such as IEEE 802.11 type of networks, which do not have any explicit signaling mechanism for link adaptation